Border Line (1992)

On a whim, the long-married Hélène decides to look up a former lover of hers. At his apartment, she is met by the man's grown son, Julien, who tells her that his father died just a few days before. Before long, she has become Julien's lover, but she is also increasingly becoming attached to the rather unlikely idea that Julien and she are genetically related. Meanwhile, her cardiologist husband cannot fathom her increasingly bizarre behavior. It is one thing to have an affair, even with a much younger man, but she seems to be edging ever-closer to the borderline between sanity and madness.